The present invention pertains to endless belts comprising a load carrier cord, and particularly to such belts wherein the cord comprises a plurality of plied yarns that are twisted together to form the cord in a direction opposite that of the first twist of the plied yarns.
In the construction of endless belts, flexible couplings, tires, and other rubber composite articles employing a load-carrying cord comprising a plurality of yarns, it is known to first twist together one or more ends of yarns in a first step to form a number of plied yarns, and to then twist together the plied yarns in a second step to form a plied or cabled cord. In such constructions it is known to twist the individual yarns in one direction in the first twist stage and to ply the twisted yarns in the opposite direction in the second twist stage. Conventionally in such constructions for such purposes, it has been found desirable to perform the twisting and plying operations to achieve a balanced twist; i.e., that arrangement which will not cause the cord to twist on itself when held in an open loop. This is accomplished for example by performing the first and the second twist operations such that the filament direction in the plied cord is the same as the longitudinal axis of the cord itself, and is described by employing equal but opposite twist multipliers in the first and the second twists.
Particularly but not exclusively in the area of multi-v-ribbed belts for utilization in modern multi-function automotive applications such as generator-starter devices, the performance requirements for both the belts' flexural fatigue resistance and its load carrying capability have increased dramatically. Higher modulus cords, including those formed of polyethylene naphthalate (PEN), poly(p-phenylene-2,6-benzobisoxazole) (PBO), aramid, or liquid crystal polymer (LCP) are known to exhibit high load carrying capability, but are known to exhibit relatively poor flexural fatigue resistance. Attempts have been made to improve the flexural fatigue resistance of belts formed from such cords, e.g., by utilizing braided cord instead of plied cord, but such methods are generally more expensive, and tend to decrease the overall cord strength, thereby diminishing the materials' value.
The need remains for a high modulus load carrying cord, including for incorporation in an endless belt or similar rubber composite article, which exhibits a good balance of load carrying capability and flexural fatigue resistance.